There has been a need for a reliable, fast, economic, and accurate method for monitoring the temperature in a wellbore for many years. Comprehensive geologic reservoir characterization (or formation evaluation) and monitoring studies require that various logging measurements be obtained in open and cased wellbores under thermally transient and/or steady-state conditions. Wellbore temperature measurements are an integral part of these studies. For example, time-lapse cased-hole temperature profiles are used in steamflood monitoring programs to determine areal and vertical sweep of steam, identify heated or cooled zones within a reservoir, and to determine reservoir heating rate, heat loss to surrounding strata, barriers to vertical flow, and steam zone pressure. In addition, time-lapse open hole temperature measurements are used in sedimentary basin hydrocarbon maturation studies to establish the time and locations at which hydrocarbon generation occurred and to identify depth limits to occurrences of commercial hydrocarbon reservoirs.
Various logging measurements such as resistivity, dielectric, carbon/oxygen, and pulsed neutron capture are sensitive to wellbore and near-wellbore temperature changes. These temperature changes can occur both spatially (i.e., radial or longitudinal temperature gradients) and temporally (e.g., under transient conditions). The significance of these effects depends on the time response characteristics and depth of investigation of the tools as well as the magnitude of the temperature changes. Consequently, resulting reservoir porosity and fluid identification data can be extremely difficult to interpret without accurate knowledge about wellbore and reservoir temperature changes that occur during the logging process. In order to minimize potential project development risks and maximize reservoir management efficiency (e.g., evaluating reservoir characteristics, monitoring reservoir performance, and optimizing operating strategies), wellbore temperatures must be included in the evaluation and interpretation of these data.
Ideally, wellbore temperature measurements are often assumed to closely represent the true formation or reservoir temperatures. In reality, however, the measurement of transient and steady-state wellbore temperatures and the extrapolation of these measurements to steady-state reservoir temperatures can be a complicated process. For example, open-hole temperature measurements are affected by large radial gradients resulting from the heating or cooling of the wellbore and surrounding reservoir by circulating drilling mud. Steady-state wellbore and reservoir temperatures can be estimated using wellbore heat transport models to extrapolate transient temperature measurements obtained during and immediately following the drilling process. However, studies show that the resulting steady-state temperature estimates are very sensitive to the accuracy of the transient wellbore measurements.
Similarly, cased-hole temperature measurements are affected by transient conditions caused by wellbore cooling and heating processes associated with cyclic fluid injection and production. In steamflooded reservoirs, steady-state wellbore temperatures can change rapidly from one depth location to another, resulting in steep thermal gradients inside the wellbore. In such cases, natural convection of wellbore fluids can significantly alter or "smear" cased-hole temperature profiles from true reservoir values. These smeared profiles can be corrected using wellbore heat transfer models in conjunction with heat flux measurements. The reliability of the corrected temperature profiles are, of course, dependent upon the accuracy of the initial "uncorrected" temperature profile measurements.
In general, fast responding temperature logging tools are crucial to obtaining reliable and accurate temperature measurements, for either open or cased-hole wells. Unfortunately, because of thermal inertia, temperature logging tools do not respond instantly to changes in environmental temperature. Instead, existing temperature logging tools are known to respond in a transient manner. The rate at which the tool reaches thermal equilibrium with its surrounding environment depends on many factors such as tool design, wellbore fluid, magnitude of the temperature change to which it is exposed, logging speed, and sensor design. Consequently, running temperature logs at a continuous speed, or with insufficient stationary time intervals can cause the tool to "thermally lag" behind the actual wellbore temperature changes.
Existing temperature logging tools are typically designed to monitor the operating temperature of other wireline logging tools for equipment diagnostic purposes. For example, maximum recording thermometers are attached to a suite of tools to monitor the highest temperatures they encounter during logging. More recently, Schlumberger.TM. has developed an Auxiliary Measurement Sonde (AMS) tool for continuously monitoring the tool temperatures during logging. Unfortunately, these temperature monitoring devices are incorporated into massive wireline sondes which are designed to prevent rapid heating or cooling of electronic components. Consequently, the temperature tools have large thermal time constants preventing quick response to wellbore temperature changes.
U.S. Pat. No. 4,811,598 (assigned to Applicant's assignee and hereby incorporated by reference) teaches a wall-contact temperature tool that improves thermal response characteristics in cased-hole wells. A relatively fast responding temperature sensor, such as a thermocouple or resistance temperature detector (RTD) or thermistor is mounted on the surface of a bow-spring centralizer, which is attached to a standard wireline logging sonde.
The method used to physically attach the temperature sensor to a bow spring or side-arm caliper has a direct impact on response time, as does the overall mass of the tool (see S. Griston, "Fluid Effects in Temperature Observation Wells", SPE Paper No. 19740, presented at the 64th Annual Technical Conference, San Antonio, Tex., Oct. 8-11, 1989). In addition, if the sensor is caliper mounted, the overall temperature response depends upon how effectively the sensor makes contact with the wellbore wall. It is therefore desirable to develop a method and apparatus to establish a reliable means of monitoring sensor contact quality.
In the wireline logging industry, it is typically assumed that wall-contact tools (including resistivity and nuclear tools) maintain complete sensor contact with the wellbore wall throughout the logging process. However, field data suggest that in rugose wellbores (often encountered during open-hole logging), both bow-spring and caliper-arm mounted temperature sensors do not consistently make complete contact with the wellbore wall. Consequently, the accuracy and reliability of the resulting measurements (which include wellbore temperature) are considerably degraded.
There are currently no adequate methods for determining the degree of contact between the sensor and the wellbore wall. Present log interpretation methods ignore the possibility of poor contact, unless it is an extreme case, such as in situations where there is a large washout. However, field data show that temperature response is highly correlatable with wellbore rugosity (the change in wellbore diameter with depth). It is also true that the response of a heat flux sensor will be highly sensitive to borehole wall contact since the sensor is specifically designed to measure heat flow through a surface.
None of the existing methods utilize a heat flux sensor in combination with a temperature sensor to correct for the effects of thermal inertia and tool response time, wellbore fluids, and inconsistent wall-contact effectiveness.
Prior work that discloses the use of downhole temperature sensors yet does not compare measurements of heat flux and temperature sensors that contact the wellbore wall with measurement of heat flux and temperature sensors that are kept in contact with drilling fluid include U.S. Pat. No. 3,981,187; U.S. Pat. No. 4,578,785; U.S. Pat. No. 4,109,717; U.S. Pat. No. 3,014,529; Soviet Patent No. 0156,504; Soviet Patent No. 0732,515; and French Patent No. 1,165,791.
Therefore, there is still a need for an improved, reliable, fast, economic, and accurate method and apparatus for monitoring the temperature in a wellbore that corrects for the effects of tool and sensor response time, wellbore fluid, and inconsistent wall-contact effectiveness.